Today, in many technical fields, electrical switching devices for switching higher electrical powers are needed. For example, powerful electric motors that have to be switched on or off are used in production machines, industrial electric vehicles and increasingly in private hybrid electric vehicles.
In the following, for illustration purposes, embodiments of the invention will be described with reference to a switching device, for instance, a high power switch used as a main battery switch in a hybrid electric vehicle. However, the invention is not so limited a may find its application in conjunction with any other type of high power electric consumer load.
Main battery switches in hybrid electric vehicles have to switch higher voltages, e.g. in the range of 100 V to 300 V at currents in the range of 100 A. Of course, in other applications, lower or higher voltages at lower or higher currents may also need to be switched.
Although, especially with regard to a hybrid electric vehicle, it is highly sensible to power down all consumer loads, and, in particular, the electric motor. However, for security reasons, a mode has to be provided for safely disconnecting the electric motor from a corresponding high power battery under full load conditions in case of an emergency.
Generally switching devices may be based on different switching components such as for instance, Metal Oxide Field Effect Transistors (MOSFETs), Insulated Gate Bipolar Transistors (IGBTs), relays etc.
MOSFETs are widely used as power semiconductor switches. However, depending on the requirements of the respective application, MOSFETs may exhibit on-state resistances that cause too high dissipation losses within the MOSFET when it is switched on.
For example, in an application using the best currently available 600 V-type MOSFETs, which typically exhibit an on-state resistance (RDSon) of 50 mΩ, a typical current of 100 A leads to a power dissipation of 500 W. Since these types of MOSFETs are typically comparatively expensive, a technical feasible parallel connection of several MOSFETs—in order to reduce the overall on-state resistance of the parallel MOSFETs, represents no practical solution with regard to costs.
IGBTs form another type of power semiconductor switches. One characteristic of IGBTs is their substantially constant on-state voltage of almost 2 V. In an application, in which a current of 100 A has to be switched, the constant on-state voltage of 2 V leads to an on-state dissipation loss of the IGBT switch of up to 200 W.
Furthermore, relays represent convenient devices for switching high electrical power. An advantage of relays is the fact that, by choosing appropriate contacts for the relay, on-state resistances of 0.5 mΩ of the respective switching device may be achieved in a cost-effective manner. For the purpose of comparison, in the above mentioned application case with a 100 A current to be switched, such a relay yields an on-state power loss of as low as 5 W.
However, relays suffer from one important disadvantage, namely an electric arc that is generated between the contacts of the relay when the relay switches higher currents at voltages above approximately 20 V.
This effect is even more severe for a relay switching DC voltages, since, in case of an application wherein a relay switches an AC. voltage, the first zero crossing of the AC voltage results in a secure extinction of the electric arc. However, in the case of the DC voltage, the electric arc will persist and will inevitably lead to the destruction of the corresponding relay.
For these or other reasons, there is a need for the present invention.